The adage about the importance of getting eight hours of sleep every night may seem trite as Americans become more overscheduled than ever. However, lack of sleep has been connected to depression, weight gain and a multitude of other conditions.

It’s also been linked to vehicle crashes — many of them fatal. Despite the increased attention on “drowsy driving,” according to the National Highway Traffic Safety Administration, about one in five fatal accidents in this country involve a drowsy driver.

The less sleep you get, the more likely you are to be involved in a crash. A recent study by the AAA Foundation for Traffic Safety found that people who have slept five to six hours in the past day are twice as likely to get into an accident as those who have had seven or more hours or sleep in the last 24 hours. Sleep specialists recommend seven to nine hours of shut-eye each night.

That crash rate doubles for drivers who have slept only four to five hours in the last 24 hours. It’s about the same as the rate for drunk drivers. The AAA study was based on data gathered by NHTSA from police and emergency medical services reports where drivers were asked how much sleep they’d had in the preceding 24 hours.

The good news for people who are able to take a nap during the day is that if you don’t get the recommended seven hours at night, you can make up for it within the next 24 hours. AAA’s director of Traffic Safety Advocacy and Research says that the important thing is to have had at least seven hours of sleep over the past day before getting behind the wheel.

Of course, no matter how much sleep you’ve had, if you’re not alert, you shouldn’t drive. Many medications make people drowsy, for example. Driving long distances without a break, especially at night, can lead to drowsiness.

If you’ve been involved in an accident caused by another driver, one of the things that investigators should determine is whether that driver was operating on too little sleep or was made drowsy, distracted or impaired by some other factor. This information can be important in any legal action to may bring against the driver.